The large edge-on spiral, the fuzzy elliptical galaxy immediately to its right, and the spiral near the top of the image are members of the group, while the small spiral galaxy exactly in the middle is a more distant background galaxy.

Galaxyclusters are found by optical or infrared telescopes by searching for overdensities, and then confirmed by finding several galaxies at a similar redshift.

Clusters are quite prominent in X-ray surveys and along with AGN are the brightest X-ray emitting extragalactic objects.

However their velocities are too large for them to remain gravitationally bound by their mutual attractions, implying the presence of an additional invisible mass component.

Since this gas is in approximate equilibrium with the overall cluster gravitational field, its distribution in the cluster traces out the overall cluster gravitational field, and therefore allows calculation of the total mass distribution in the cluster.

Note: clusters of galaxies should not be confused with starclusters such as globular clusters and open clusters, which are structures within galaxies.

Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years.

Intergalactic space, the space between galaxies, is a near vacuum with an average density of less than one atom per cubic meter of gas or dust.

Structures containing up to about 50 galaxies are called groups of galaxies, and larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters.

www.biocrawler.com /encyclopedia/Galaxy (1445 words)

Galaxy(Site not responding. Last check: 2007-10-22)

Stars are almost always found in collections called galaxies, together with gas, dust, and "dark matter"; ~10-20% of a galaxy is composed of stars, gas, and dust.

Galaxies are held together by gravitational attraction and the galactic components orbit a common centre.

Galaxy is also a type of chocolate candy bar made by the Mars company.

The formation of galaxies is still one of the most active research areas in astrophysics; and, to some extent, this is also true for galaxy evolution.

Hence the distribution of galaxies is closely related to the physics of the early universe.

Further examples of satellite dwarf galaxies that are in the process of merging with the Milky Way are the Canis Major Dwarf Galaxy, discovered in 2003 and thought to be responsible for the Monoceros Ring, and the Virgo Stellar Stream, discovered in 2005.

Galaxy formation and evolution: the study of the formation of the galaxies, and their evolution.

The existence of our galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe seen in the recession of most galaxies from us.

That the universe remains largely made up of these elements -- hydrogen and helium -- is considered strong evidence of the validity of the big bang model.

These explosions, called supernovae, are important to the evolution of galaxies because they distribute all the common elements such as oxygen, carbon, nitrogen, calcium, and iron into interstellar space.

This means that, in the future, gravitationally bound clusters of galaxies will survive but galaxies in general will fly ever more rapidly apart.

Seyfert's Sextet is a group of galaxies in which gravitational forces are exerted between its members.

This group of galaxies lies in the head portion of the split constellation Serpens.

Another is not a galaxy in its own right but rather a string of stars torn loose from one of the other galaxies by gravitational pull.

www.factbook.org /wikipedia/en/s/se/seyfert_s_sextet.html (153 words)

Discover the Wisdom of Mankind on Galaxies(Site not responding. Last check: 2007-10-22)

Clusters of galaxies are often dominated by a single giant elliptical galaxy, which over time tidally destroys its satellite galaxies and adds their mass to its own.

Superclusters are giant collections containing tens of thousands of galaxies, found in clusters, groups and sometimes individually; at the superclusterscale, galaxies are arranged into sheets and filaments surrounding vast empty voids.

In 1999 the Hubble Space Telescope estimated that there were 125 billion galaxies in the universe, and recently with the new camera HST has observed 3000 visible galaxies, which is twice as much as they observed before with the old camera.

The presence of highly ionized oxygen between the galaxies implies there are huge quantities of hydrogen in the universe, which is so hot it escapes detection by normal observational techniques.

Clusters of galaxies form where the filaments intersect.

The oxygen "tracer" was probably created when exploding stars in galaxies spewed the oxygen (created in their cores through nuclear fusion) back into intergalactic space where it mixed with the hydrogen and then was shocked and heated to temperatures over 360,000 degrees Fahrenheit (100,000 degrees Kelvin).

origins.jpl.nasa.gov /library/story/050300-a.html (744 words)

Science Results Using HEASARC Data(Site not responding. Last check: 2007-10-22)

The determination of cluster properties and their evolution provides fundamental insights into the formation of largescalestructure in the universe.

Further, the masses and scales of clusters are such that they should comprise a fair sample of the mass of the Universe, with representative fractions of the different components.

All clusters were found to be non-isothermal, with spatial temperature variations (apart from cooling flows) of a factor of 1.3--2.

If these confining magnetic structures are sufficiently stable, and can be embedded in a background of shear flow, they are interesting candidate for a largescale confinement mechanism for molecular cloud complexes.

A statistical cluster analysis algorithm applied to poles derived from published proper motion data for 38 globular clusters and six satellite galaxies indicates at least one significant dynamical family that may be fossil evidence for a prior Galactic accretion event.

Audano (graduate student) and Sarazin are calculating the gamma ray emission expected from the population of cosmic ray particles in clusters of galaxies.

www.astro.virginia.edu /research/aas98 (8195 words)

Astronomy and astrophysics - Piclab(Site not responding. Last check: 2007-10-22)

Cosmology, a discipline that has a large intersection with astronomy, made huge advances during the 20th century, with the model of the hot big bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation,Hubble's Law and cosmological abundances of elements.

All the previous disciplines are based on the detection of photons, but we also receive information from outside the earth carried by cosmic rays, neutrinos, and, in the near future, gravitational waves (see LIGO and LISA).

Images of galaxyclusters that form a three-dimensional network called "the large-scale structure of the universe," are drawn from a database of 45,000 galaxies developed by R. Brent Tully, professor of astronomy at the University of Hawaii.

Another new exhibit, "Scales of the Universe," installed on a balcony surrounding the sphere, compares the smallest objects in the cosmos to the largest.

At one stop on the scale, the sphere represents the supergiant star Rigel in the constellation Orion while the a ball, small enough to fit into two cupped hands, represents the Sun of our own solar system.

Jupiter's largemass and high gravity attract much of the material in its region, leaving the asteroid belt too depleted in material to form a planet, and making Mars lighter in mass than Earth.

Fullerenes, which contain at least 60 carbon atoms and have a structure resembling a soccer ball or a geodesic dome, are named for Buckminster Fuller, inventor of the geodesic dome.

They looked at tooth size, tooth shape, tooth enamel structure, dental microwear and mandibular biomechanics of the fossils, most of which date back to a time before stone tools, before culture and before meat was introduced to the diet.

Stars were found to be far away objects, and with the advent of spectroscopy it was proved that they were similar to our own sun, but with a wide range of temperatures, masses and sizes.

Cosmology, a discipline that has a large intersection with astronomy, made huge advances during the 20th century, with the model of the hot big bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's Law and cosmological abundances of elements.

For a more detailed history of astronomy, see the history of astronomy.